There exist multiple types of light sources currently in use for providing illumination. Such light sources are commonly referred to as lamps. Most of the lamps in use are electrically powered. One of the most common types in use is an incandescent lamp in which a filament of tungsten or other refractory material is heated by the power dissipated in the electrical resistance of the filament when an electrical current is forced through it. Usually, the electrical current is supplied to the filament directly from a power line providing a more or less constant average alternating-current voltage or from a power supply or battery operating at a more or less constant direct-current voltage. Incandescent lamps are designed to operate at voltages typically in the range between a few volts to 250 volts. Much of the dissipated power is radiated as heat in the form of infrared radiation, some of the power converts to heat that leaves the lamp through thermal conduction and convection, and a relatively small portion of the power is radiated as visible light. For an incandescent lamp the power efficiency of the lamp, which is calculated as the ratio of the power radiated as visible light to the total electrical power dissipated in the lamp, is typically about 5 percent or lower.
Another common type of lamp is a discharge lamp, in which electrical current flows through a gas. Excited by the current, the gas emits infrared, visible, and ultraviolet radiation. A fluorescent lamp is a type of discharge lamp in which much of the ultraviolet radiation is converted to visible radiation by a fluorescent coating. Other types of discharge lamps include sodium lamps, carbon arc lamps, mercury arc lamps, neon lamps, xenon lamps, and metal halide lamps. Visible light is radiated with power efficiencies ranging up to the low twenty percent range. Much of the remaining power is dissipated as infrared or ultraviolet radiation, and some may be converted to heat that is carried away through thermal conduction and convection.
Unlike incandescent lamps, discharge lamps generally require ballasts or controlled-current sources for stable operation. The operating voltage of a discharge lamp is frequently in the range of operating voltages of incandescent lamps, but the current through the lamp is much more sensitive to the voltage. Operation directly from an unregulated voltage supply such as a battery or an alternating-current power line may result in malfunction of the lamp due to large variations in current and, hence, power dissipation as the supply voltage varies.
A newer category of light sources distinct from incandescent lamps and discharge lamps is that of solid-state light-emitting devices. Included in this category are, for example, electroluminescent devices, semiconductor lasers, and light-emitting diodes. Unlike incandescent lamps and discharge lamps, solid-state light-emitting devices suitable for illumination emit substantially all of their radiation in the form of visible light, and the amount of power emitted in the form of infrared or ultraviolet radiation is relatively insignificant. Currently, the most efficient of these solid-state light-emitting devices, the light-emitting diodes (LEDs) and the semiconductor lasers, may operate at power efficiencies as high as twenty to forty percent. The electrical power that is not converted to light is converted to heat. Due to the small sizes of practical high-power devices, usually only a small fraction of the heat is removed through convection, and the remainder of the heat is removed through thermal conduction. Relative to incandescent lamps and discharge lamps in general, the efficiency and reliability of solid-state light-emitting devices are more sensitive to temperature. The efficiency drops significantly at high operating temperatures, and the rate at which the light output degrades over time increases by a factor of typically between two and ten for every 10° C. rise in temperature. A heat sink and a thermally conductive path between the light-emitting device and the heat sink are generally provided in order to limit the rise in temperature of the light-emitting device due to the heat generated within it. For example, LEDs are frequently furnished by the manufacturer as surface-mount assemblies that may be soldered to conductors on the top surface of a thin electrically insulating circuit board backed by a sheet of thermally conductive metal such as aluminum or copper. Metalized conductive vias in the insulating circuit board may assist in conducting heat from the conductors on the top to the metal sheet on the back.
The most efficient solid-state light-emitting devices, the LEDs and semiconductor lasers, are generally limited by practical considerations to input power levels of a few watts or lower per device. Each device runs at a voltage typically between two and four volts. For applications such as wide-area illumination that require input power levels of tens to hundreds of watts, it is common practice to include multiple light-emitting devices in an assembly and to electrically connect the multiple light-emitting devices in series. Given a fixed operating current and temperature, the total light output of such a series-connected assembly and the voltage across the assembly are both proportional to the number of light-emitting devices in the assembly.
Most incandescent lamps and discharge lamps are hermetically sealed, since they require the maintenance of a partial vacuum, but solid-state light-emitting devices typically are not hermetically sealed. As a result, special considerations may apply regarding the protection from the environment of an assembly of LEDs or semiconductor lasers and the conductors that interconnect them and carry electrical power to them. In particular, liquids coming into contact with the conductors or the light-emitting devices, especially while electrical power is applied, can result in electrolytic corrosion of the conductors or of the light-emitting devices leading to premature failure of the assembly. Human contact with the conductors or with liquids in contact with the conductors can result in electrical shock. Mechanical stress on one or more conductors, wires, or cables exiting the assembly may result in damage to the assembly, if the conductors, wires, or cables are not sufficiently secured mechanically to the assembly.
If portions of a lamp assembly employing light-emitting devices should intercept and cause to be absorbed some of the light emitted by the light-emitting devices, the photonic power efficiency of the lamp assembly may be reduced. This fact is a consideration influencing the design of various portions of the assembly including any that provide environmental protection or that contribute to the mechanical or electrical connections within the assembly.
High-power light-emitting devices suitable for use in illumination applications can be bright enough to cause eye damage in some circumstances. For some applications eye safety may be a consideration in the design of a lamp assembly.
Unlike incandescent lamps and discharge lamps, which may radiate light in almost all directions, solid-state light-emitting devices usually radiate in some directions but not others. An LED, for example, typically radiates with a Lambertian pattern into the space on one side of a plane. Special considerations may apply, therefore, to the way light-emitting devices are oriented within an assembly or the way an assembly is oriented while the assembly is being applied to provide illumination.
Each solid-state light-emitting device has its own spectral characteristic, which is defined by the distribution of power of the light emitted over the wavelength of the light emitted. For some the spectral characteristics show distributions in which most of the emitted power is confined to a narrow wavelength range. The light from these devices has a highly saturated color, the color depending on the dominant wavelength. Other devices may emit light that is less saturated in color or that is white. These devices have spectral characteristics with broader distributions over wavelength. No one solid-state light-emitting device has yet been devised that has a spectral characteristic broad enough to match that of the sun. When a broad spectral characteristic is desired, the light from multiple light-emitting devices of different spectral characteristics or colors is frequently combined. In applying solid-state light-emitting devices in illumination applications it is generally the practice to blend the light from these multiple devices in a way that prevents observers from perceiving the separate colors of the devices. The light from this source consisting of multiple light-emitting devices of different colors then appears as light of a single uniform spectral characteristic or color.
As is the case for discharge lamps, the electrical current drawn by solid-state light-emitting devices is usually so sensitive to the voltage across them that some form of ballast or current limiting in the power supply is desirable to prevent excessive variations in the power supplied to the devices as a result of normal variations in voltage on the power source. This problem exists with series-connected devices to the same extent that it does with individual devices. Common practices include the use of a resistor electrically in series with the light-emitting devices, use of a ballast inductor in series with an alternating current source supplying power to the light-emitting devices, or use of a switching-mode power converter to drive the light-emitting devices with a regulated current.